1. Field of the Invention
The present invention relates to an integrated energy management system for refrigeration systems and more particularly to a method and system for automatically controlling certain functions of a refrigeration system to minimize the energy costs and to provide the user with continuous data on parameters of the system.
2. Description of the Prior Art
Over the recent decades, the use of refrigeration systems and air conditioning systems has grown to the point that such systems represent a major consumer of electrical energy in the United States as well as in many of the major industrial nations of the world. The designs for such systems developed during a period when electrical energy was extremely cheap. Efficiency often was relegated to a secondary factor in such designs with the initial cost being a primary factor. However, in recent years, the shortages of fossil fuels for producing electrical energy and the continuing increase in demand for electrical energy has led to significant improvements in the design to minimize input power, both in the interest of conservation of energy and the reduction of operating costs.
As is well known, the price of electricity has continued to spiral upward to the point that operating costs may dominate over capital investment costs. This situation is particularly severe with respect to large industrial refrigeration systems, such as those having capacities from 100 tons to 1,000 tons and greater. To illustrate, a 1,000 ton refrigeration system, designed to the state of the art, may have an annual operating cost of over $400,000.00 for power at 5.cent. kilowatt hour. This number assumes that the system is operating at its design parameters and that all of the components are operating at their maximum efficiencies.
What may not be generally realized is the penalty that the user must pay for naturally occuring deterioration of the elements of the system and for the changes in the operating environment which causes the operating parameters to depart from the original design parameters. To illustrate this problem, assume an ammonia compressive type refrigeration system having an evaporative condenser and the following design parameters:
______________________________________ Average Load 1000 Tons Suction refrigerant 20.degree. F. temperature Wet bulb temperature 78.degree. F. Condensing temperature 96.3.degree. F. Compressor motor efficiency 0.92 Condenser fan and water pump 0.85 motor efficiency Fan and water pump horsepower 10% of tonnage load ______________________________________
When this system is operating at its design parameters and at maximum efficiency, with a power cost of 5.cent. kilowatt hour, the cost per ton of cooling may be calculated to be 4.5.cent. per ton per hour which gives an annual operating cost of $394,200.00.
Two common problems that arise that will decrease the efficiency and therefore increase the operating costs are: build up of non-condensible gases in the system; and film fouling in the evaporative condensers. As is well known, either of these problems reduces the efficiency of the system. For example, a fouled condenser will result in an increase in condensing temperature and head pressure. Assuming that this change is from 96.3.degree. F. to 101.degree. F., it can be calculated that the costs per ton per hour will increase to 4.78.cent. and the yearly cost to operate will increase to $418,728.00, which represents wasted power totalling $24,528.00. In this same system, for power costs of 7.cent. per kilowatt hour, the wasted power would cost $34,339.00 if the conditions were allowed to continue for one year.
Similarly, if non-condensibles build up in the system the head pressure will increase for a given load. Assuming that the normal pressure for 96.3.degree. F. condensing temperature to 185 psi increases to 201 psi due to non-condensible gases, the efficiency is decreased about 8% causing a yearly increase in power cost of $31,536.00.
It may be noted that in the example given, a condensing temperature of 96.3.degree. F. will occur for ammonia at a head pressure of 185 psi under normal conditions. When the condensing temperature rose to 101.3.degree. F. as assumed due to a fouled condenser, the head pressure would also go up proportionally. However, most systems in current operation provide the operator only with measurements of the head pressure and many plant operators depend upon such pressure readings to determine when certain preventive maintenance is required such as purging the system of non-condensible gases and cleaning of condensers. As will be shown in more detail later, to determine when condensers are fouled requires knowledge of the wet bulb temperature at the condenser and the condensing temperature of the refrigerant. Similarly, determination of the presence of non-condensible gases requires knowledge of the head pressure and the condensing temperature. As is well known, the head pressure and the condensing temperature can increase in synchronism in a system operating at its maximum efficiency due to an increased load. Similarly, when operating on a less than maximum load, the head pressure and condensing temperature will decrease proportionately. Thus, a head pressure reading alone cannot indicate a need for purging or for maintenance of the condenser. Consequently, many large systems may operate for an extended period of time with these problems remaining undetected and therefore waste power.
Another source of wasted energy in present systems stems from the problems of controlling the fans and water pumps associated with the condensers. For example, when the relative humidity is much lower than the utilized as a design parameter, it may not be necessary to operate the fans since natural evaporation may be sufficient. The important parameter here is the difference in temperature between the wet bulb reading and the condensing temperature referred to as .DELTA.T.sub.c. For the design example given above with a wet bulb temperature of 78.degree. F. and a condensing temperature of 96.degree. F., the .DELTA.T.sub.c is equal to 18.degree. F. Under conditions of much lower temperature than the average value for the location of the system utilized at design, the wet bulb temperature will go down. However, the control criteria should be .DELTA.T and not an absolute measurement of pressure or humidity, since a higher condensing temperature will also increase .DELTA.T.sub.c, but the wet bulb temperature remains constant. Similarly, if the load drops on the refrigeration system the cost to operate the compressors will drop proportionately but the condenser pumps and fans will remain operating at full load. Again, power may be wasted from this source.
When the outside air temperature is very low and at periods in which the water may freeze, the water pump motors may be cut off, saving energy. Again, the wet bulb and dry bulb temperatures must be known to determine when this can be done.
If a user of this type of refrigeration system can keep the system free of non-condensible gases, maintain the condenser at maximum efficiency, and control the running of the water pump motors and fan motors, the cost per ton per hour of the system can be prevented from increasing above its value predicted from the design and can in many instances be significantly reduced below that by taking advantage of favorable changes in ambient conditions and during periods of reduced load. These various problems have been recognized by those of skill in this art. For example, in the following U.S. patents, the head pressure has been used as an indicator of efficiency and apparatus provided sensitive to a measurement of the head pressure to effect certain system controls: Vogel, et al, U.S. Pat. No. 4,193,781; Seely, U.S. Pat. No. 3,481,152; and Wood, U.S. Pat. No. 3,196,629. Vogel has elected to prevent the system head pressure from going below a minimum for a particular operating mode. Wood is concerned with a refrigeration system using an air cooled condenser to maintain a desired head pressure to prevent the capacity of the system from being impaired during low outside ambient temperature conditions. Seely utilizes a plurality of condensers, switching the condensers on and off to maintain a desired head pressure. Therefore, these patents fall far short of the requirements delineated above. In U.S. Pat. No. 4,085,594 to Mayer, a control responsive to the temperature of the condensed refrigerant is disclosed to vary the speed of cooling tower fans so that the minimum power is required at all times. While possibly advantageous in a tower which simply cools the water, it does not take into account the relative humidity as is desirable in an evaporative condenser. McAshan, U.S. Pat. No. 3,707,851 measures the ambient air temperature in a refrigeration condenser and the temperature of the liquid refrigerant to produce an alarm when a malfunction in the system, fouled condenser, or the like occurs. Carroway, in U.S. Pat. No. 2,847,831, is concerned with controlling the operation of systems, including evaporative condensers, and recognizes the conditions under which the fan and/or pumps can be shut down. Various mechanical type pressure and temperature measuring elements are utilized, but he chooses to base his control on atmospheric air temperature rather than wet bulb temperature.
Automatic purge mechanisms are known in the art which will detect the presence of non-condensible gases in the system and will automatically vent these to the atmosphere. U.S. Pats. to Indis, et al, No. 3,013,404 and to Kieme, No. 2,598,799, are examples. However, automatic purging in an ammonia system which results in loss of refrigerant causes safety problems. It is thus preferable to require purging to be done manually under controlled conditions. When non-condensibles are due to leaks, the automatic purgers may not operate fast enough and therefore are not satisfactory. Other patents have provided various display and diagnostic aids to assist the operator in locating malfunctions in a system. These include Anderson, et al., U.S. Pat. No. 4,038,061 and Wills, U.S. Pat. Nos. 4,146,085. Schulze, Sr., in U.S. Pat. No. 4,186,563, discloses a display of energy use. Kayan, in U.S. Pat. No. 3,082,951, discloses methods of calculating the performance of a refrigeration system from system measurements.
Although some of the energy efficiency factors discussed above have been attacked individually by workers in this art, there has been no known modern and efficient energy management system available which can provide certain automatic controls responsive to measured values of specific system parameters, and which also provides readouts to the operator or alarms alerting the operator to perform certain manual maintenance actions.